Stress relief bend useful in an integrated circuit redistribution patch

ABSTRACT

A metallic or an electrical trace having a terminus and a stress relief bend formed in the trace adjacent the terminus. The electrical trace may have a portion carried by a flexible substrate to form a flexible circuit. The stress relief bend may be free floating and extend from the flexible substrate or may be encapsulated by the flexible substrate. The electrical circuit and the flexible circuit each have a generally planar portion extending in the X and Y axis, with the stress relief bend projecting into the Z axis. This allows electrical traces to be spaced with a very narrow pitch because the stress relief bend does not consume any valuable real estate on the flexible circuit or the substrate to which the electrical trace is applied.

RELATED APPLICATION

[0001] This is a continuation of U.S. patent application Ser. No.08/959,837 filed Oct. 29, 1997.

TECHNICAL FIELD

[0002] The invention relates to a metallic or an electrical leadterminus with a stress relief portion.

BACKGROUND OF THE INVENTION

[0003] A long standing problem know to those skilled in the electronicsart involves the mismatch between the thermal properties of materialmaking up an electronic device and the materials making up the substrateto which the device is mounted. Canestaro at al U.S. Pat. No. 4,728,751addressed the mismatch problem associated with mounting an electronicdevice, such as a semiconductor chip made primarily of silicon, directlyonto an organic substrates, such as printed circuit board made primarilyof glass cloth impregnated with epoxy resin or other suitable materials.Canestaro et al solved the mismatch problem by processing a selectedsurface portion of a substrate to form a relatively low adhesive areasurrounded by a relatively high adhesive area. A circuit line patternwas deposited on the substrate, including circuit lines having terminiwith stress relief bends, located in the relatively low adhesive area onthe surface of the substrate. As such, the terminus of each of thecircuit lines floats on the surface of the substrate while the remainderof the circuit line is fixed to the relatively high adhesive area on thesurface of the substrate. However, the circuit line including the reliefbend are co-planar and parallel to the surface of the substrate.Consequently, the lines with the stress relief bend consume asubstantial amount of real estate on the substrate.

[0004] Ashby, U.S. Pat. No. 3,519,890 also attempted to solve thethermal mismatch problem by interconnecting a microelectronic circuitcontaining die with a substrate utilizing a meandering conductive lineto relieve the stress on the die and conductive line under thermalconditions and temperature cycling. However, the meandering lines didnot have a common geometry and as a result took up a substantial amountf valuable area on the mounting substrate.

[0005] The present invention provides alternatives to and advantagesover the prior art.

SUMMARY OF THE INVENTION

[0006] The invention includes a metallic or an electrical trace having aterminus and stress relief bend formed in the trace adjacent theterminus. The stress relief bend is a shaped geometry that has anincreased length to distribute strain. The geometry of the bend has theeffect of changing the overall elasticity of the trace section (comparedto linear sections) thereby limiting strain. The electrical trace mayhave a portion carried by a flexible substrate to form a flexiblecircuit. The stress relief bend may be free floating and may extend fromthe flexible substrate or may be encapsulated by the flexible substrateor other material. The electrical trace and the flexible substrate eachhave a generally planar portion (extending in the X and Y axes), withthe stress relief bend projecting from the plane (along the Z axis).This allows traces to be spaced with a very narrow pitch because thestress relief bend does not take up any valuable real estate on theflexible circuit or the substrate to which the trace is applied.

[0007] In the preferred embodiment, the invention included an integratedcircuit redistribution patch including an electrical trace having aterminus and stress relief bend adjacent the terminus. The flexiblecircuit included raised electrical contact features in addition to thoseutilized to make the termination to the integrated circuit chip. Theadditional raised electrical contact features allow for electricalconnection to input and output (I/O) termination pads on the peripheryof the integrated circuit and a higher interconnection site capable ofdistributing stress.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 illustrates a redistribution patch including a stressrelief bend according to the present invention;

[0009]FIG. 2 is an enlarged view of a redistribution patch including astress relief bend that has been encapsulated by a dielectric materialaccording to the present invention;

[0010] FIGS. 3A-3D illustrate tools for forming dimples and dents in amandrel to make a metallic trace having raised features and stressrelief bends according to the present invention;

[0011]FIG. 4 illustrates a mandrel for making a metallic trace havingraised features and stress relief bends according to the presentinvention;

[0012] FIGS. 5-9 illustrate a process for making a metallic trace havingraised features and stress relief bends and carried by a flexiblesubstrate according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013]FIG. 1 illustrates a metallic or an electrical trace 10 having aterminus 12 and a stress relief bend 14 according to the presentinvention. In the embodiment shown in FIG. 1, The electrical trace 10with stress relief bend 14 is utilized in an integrated circuitredistribution patch. The patch is used to redistribute, or fan in,peripheral input/output leads over the face of the integrated circuitchip. The combination includes an integrated circuit chip 16 having acontact pad 18 formed on an upper surface thereof. A flexible circuit 20is positioned over the integrated circuit chip 16 with a complaint layeror an elastomer pad 22 interposed between the chip 16 and flexiblecircuit 20. The flexible circuit 20 includes a lower and upperinsulation layer 24, 26 selectively deposited to encapsulated or exposevarious portions of the electrical trace. The insulation layers 24, 26may be a dielectric such as a polyimide. The terminus 12 of theelectrical trace may be a flat but preferably is a raised feature 30 andis sonic welded, soldered 28 or attached by an electrically conductiveadhesive to a contact pad 18 on the integrated circuit chip. The raisedfeature 30 may be utilized to make electrical contact to an electricalcomponents such as an integrated circuit chip or the raised feature maybe utilized to make thermal contact to a heat generating component.

[0014] The stress relief bend 14 is formed in the trace adjacent to theraised feature 30. Preferably, the stress relief bend 14 is surroundedby a first and second substantially planar portion 32, 34 of theelectrical trace. The stress relief bend 14 may be free floating whenextended outwardly from the insulation layers 24, 26 as illustrated inFIG. 1. Alternatively, the stress relief bend 14 may be encapsulated bythe insulation layers 24, 26 as illustrated in FIG. 2. The electricaltrace may include a second raised feature 35 extending in an oppositedirection to that of the first raised feature 30 (terminus) for makingcontact to the base substrate. Preferably the first raised feature 30,stress relief bend 14, and second raised feature 35 are all apart of ametallic trace which includes a continuous layer of conductive materialsuch as copper formed by a mandrel process to be described hereafter.The elastomer pad or complaint layer 22 prevents damage to the secondraised feature 35 or accommodates surface irregularities when the secondraised features engage another component such as a test socket orsurface mount solder interconnection on a multilayer printed wiringboard.

[0015] As will be appreciated from FIG. 1, the electrical trace 10 andflat flexible circuit 20 each have a generally planar portion includingportion 32, 34, with the stress relief bend 14 extending from the plane.Thus, the stress relief bend 14 does not take up any valuable realestate on the flexible circuit or the substrate to which the trace isapplied. The term stress relief bend as used herein includes a shapedgeometry that has increased length to distribute strain. The geometryhas the effect of change the overall elasticity of the trace section(compared to linear sections) thereby limiting strain. The bend isnonlinear having a portion extending out of the plane that is at leastU-shaped, V-shaped, or has a wave configuration, or any otherconfiguration which relieves stress on the electrical trace and thesonic weld or solder used to secure the terminus to an electricalcomponent or heat generating component during operation or thermalcycling.

[0016] An electrical trace having raised features 30, 35 and stressrelief bends 14 may be manufactured by a variety of methods includingmechanically forming conical shaped raised features and stress reliefbends using a tool forced into a flat conductive trace. Preferably, theelectrical trace is formed using a mandrel method similar to thatdisclosed in Crumly et al U.S. Pat. No. 5,207,887 the disclosure ofwhich is hereby incorporated by reference.

[0017] According to the present invention, a mandrel for making flexiblecircuits is fabricated from a stainless steel shim or foil stock. FIG.3A illustrates a tool for making a U-shaped dent (having a relativelylarge radius) in the foil stock corresponding to a stress relief bend 14in the electrical trace. Likewise, FIG. 3B illustrates a tool for makinga V-shaped dent (having a relatively narrow radius) in the foil stockcorresponding to a different shaped stress relief bend 14 in theelectrical trace. FIG. 3C illustrates a tool for making a multiple wavein the mandrel corresponding to a stress relief bend. FIG. 3Dillustrates a tool for making conical shaped or hemispherical dimples inthe foil stock corresponding to conical shaped or hemispherical raisedelectrical contact features 30, 35 in the electrical trace. Utilizingthe above described tools, dimples 30′ and dents 14′ are formed in afirst face of the shim stock corresponding to the first raised feature30 and bend 14 by placing a second face of the stock on an adhesivelayer overlying a piece of soft metal, not shown, such as aluminum andforcing the above described tools into the stock and underlying softmetal or other suitable substrate. The thickness of the shim or foilstock is chosen so that when the first face is dimpled or dented, thesecond face is deformed so that bumps or protrusions 35′ are formedextending out of the second face corresponding to the second raisedfeature 35. Inwardly facing dimple is formed by turning the shim stockover and dimpling the second face in a similar manner. As such, a dimpleis formed in the second face and a bumps extending upwardly from thefirst face of the foil stock. This allows for the raised electricalcontact features 30, 35 to be formed so as to extend outwardly form thefaces of the flexible circuit in opposite directions, as illustrated inFIGS. 1-2. The adhesive layers are thereafter removed by dissolving withan aqueous solvent to provide a formed mandrel 36 as illustrated in FIG.4.

[0018] The shim stock typically is from 2-4 mils thick and can be flashplated with copper and circuit constructed thereupon in a manner to bedescribed hereafter. The transfer cover has to have a hole to clear theupwardly facing bumps and/or protrusions. The downwardly extending bumpsor protrusions will be slightly larger than the tool dimensions. Oncethe outwardly facing bumps and protrusions are formed, a layer ofadhesive 38 is applies to the shim stock laminated onto a convenientsubstrate 40 such as a polyimide fiberglass board or a flexiblesubstrate such as polyimide to form a more suitable size mandrel, asillustrated in FIG. 5. The inwardly extending bumps and protrusions pushinto the substrate 40 and at least a portion of the depressions in thesecond face is filled with the heated substrate material. However, dueto the strength of the foil there is no need to back fill thedepressions. The substrate material may be laminated at temperaturesranging from 375 to 400 degrees F and at pressures of 275 to 325 psi.

[0019] As illustrated in FIG. 5, the mandrel 36 including the dimples,dents and bumps is then coated with a copper coating 42, typicallyreferred to as flash plated, which covers the entire surface of themandrel. The flash plated copper 42 is applied by electroplating orother known techniques. The thin conductive coating presents theadhesive that will be used to laminate the circuit from adhering to themandrel surface. Flash plating is a conventional electrolytic platingformed in a very short or momentary operation so that for only a seriesthin plating coat is provided. The flash plated coat is very thincompared to the thickness of the electrical circuit traces that are tobe made. For example, for a circuit trace of 1 ½ mil thickness, a flashplating of copper on the mandrel will have a thickness of 0.01 to 0.02mils. The thin flash plate 42 is employed because it can be relativelyeasily released from the stainless steel mandrel by flash etching.Obviously, other methods of coating the mandrel with a thin coat ofconductive material that is easily separated from the mandrel and fromthe completed circuit traces may be employed in the place of theelectrolytic flash plating.

[0020] The flash plated copper 42 is then coated with a photoresist 44,which is then optically exposed through a mask defined defining apattern of the desired circuit and developed. The photoresist that hasnot been polymerized is then removed to leave the partially completedassembly. The flash plated copper 42 coating now bears a pattern ofphotoresist 44 that is a negative pattern of the circuit trace patternto be fabricated with the mandrel.

[0021] The mandrel assembly of FIG. 6 is then subjected to a suitableadditive electroforming process, to plate up copper traces 10 over theshim bump and into the shim dimples and dents as shown in FIG. 7. Thecopper 42 coating that are not covered by the negative pattern of thedeveloped photoresist 44. Thus the plating process simultaneously formsboth the circuit traces and the raised electrical contact features 30,35 and stress relief bend 14.

[0022] The photoresist 44 is then stripped to leave the circuit tracesand the flexible raised electrical contact features on the flash platedcopper coating which is still on the mandrel as shown in FIG. 8. Theflexible circuit raised electrical contact features are partially hollowhaving a depression on an under side corresponding to the generallyconical shaped of the raised feature. If deemed necessary or desirable,the depression formed in the electroplating raised feature may be filledwith a solid material such as an epoxy. The depression may also befilled with the adhesive used to adhere the dielectric layer describedhereafter. Now a layer of suitable dielectric 26 and adhesive, such as,a layer of Kapton™, available from DuPont, and adhesive are laminated tothe mandrel assembly with the traces and circuit features under suitablehigh temperatures and pressures. Only that side of the traces and padsthat is directly in contact with the flash plated copper on the mandrelis not laminated by the adhesive/Kapton™ substrate. The assembly shownin FIG. 8, including the flash plated copper, traces, raised features,bend and the adhesive/Kapton™ substrate is removed form the mandrel.

[0023] Then, as illustrated in FIG. 9, the flash plated copper coatingwhich covers the entire lower surface of the assembly is removed by aflash etching process. Thereafter, a second dielectric layer 24 such asKapton™ is applied to the other side of the electrical trace 10.Although the invention has been described heretofore as an electricaltrace with a stress relief bend formed therein, the invention isapplicable to a metal trace for making connection to a heat generatingdevice and dissipating heat through the metal trace. Such metal tracesare also soldered or sonic welded to the heat generating device andsuffer from the same thermal mismatch problem.

1. A product comprising: a metallic trace having a first and a secondsubstantially planar potion and a raised portion extending from thefirst substantially planar portion and secured to a first component, astress relief bend formed in the metallic trace adjacent the securedraised portion, the stress relief bend extending from and surrounded bythe first and the second substantially planar portion of the metallictrace, and at least a portion of the metallic trace being encapsulatedby a dielectric material.
 2. A product as set forth in claim 1 whereinthe component comprises a semi-conductor device.
 3. A product as setforth in claim 1 wherein the component comprises a heat generatingcomponent.
 4. A product as set forth in claim 1 wherein the raisedfeature has a conical shape.
 5. A product as defined in claim 1 whereinthe stress relief bend is encapsulated by the dielectric material.
 6. Aproduct as set forth in claim 1 wherein at least a portion of themetallic trace is encapsulated by a flexible substrate.
 7. A productcomprising: an electrical trace having a planar portion and a raisedportion extending from the planar portion and secured to an electricalcomponent, and a stress relief bend adjacent the secured raised portionand extending from the planar portion, the raised portion having aconical or hemispherical shape.
 8. A product as set forth in claim 9wherein the electrical component comprises a semi-conductor device.